Total hip templating using a marker ball (vs. 21% magnification): no difference in accuracy but increase in potential harm

Background: Digital templating for total hip arthroplasty (THA) can help plan the surgical procedure. Magnification marker balls (MB), placed at the time of a digital radiograph have become an acceptable technique to assist with templating despite lack of evidence that they add accuracy. We compared templating accuracy using a standard 21% magnification technique versus a MB. Methods: Two blinded evaluators independently performed retrospective templating on 50 consecutive THA patients. Inter-rater reliability was assessed with interclass correlation coefficients and did not differ across techniques. Accuracy between each templating technique and final implant size was evaluated using Pearson correlations, paired t-tests, and McNemar’s tests. Results: Correlations between final implant size and template estimates were significant and did not differ between techniques. Absolute differences between final size and template estimates also did not differ. Both techniques had equal accuracy within one size of implant size: acetabulum: 62% versus 66%, femur: 78% versus 72%. When differences between templating estimates and actual sizes were examined more minutely, MB yielded a greater rate of 1-size and 2-size larger errors, while 21% magnification yielded a higher rate of 1-size smaller errors. Conclusions: Digital templating using 21% magnification or MB were equally accurate. However, when finer discriminations were made between templating estimates and the final actual size, 21% magnification had better directional accuracy and the advantage of improved clinic efficiency. Using MB, the components tended to be over-estimated, particularly femoral components, which could have a substantial negative impact, especially if the surgeon is trying to match implant with template size.